1. The Field of the Invention
The invention relates to metal matrix coated fiber composites and methods of manufacturing such composites. More particularly, the present invention provides a fiber coating which allows ceramic or metal fibers to be wet by molten metals without degradation of the fiber by the coating itself or the metal matrix. The coated fibers may be infiltrated with the metal matrix resulting in composites having improved properties such as strength, stiffness (elastic modulus), wear resistance, hardness, thermal expansion coefficient, or thermal conductivity not obtainable in pure materials.
2. Technology Review
A. Metal Matrix Composites
A "composite" is a material consisting of one phase dispersed in a continuous matrix. Composites are commonly divided into three classes, based on the types of materials used for the matrix: polymers, metals, and ceramics.
In the field of metal matrix composites, a metal matrix is reinforced with particulates or fibers to improve properties such as strength, stiffness (elastic modulus), wear resistance, hardness, thermal expansion coefficient, or thermal conductivity. Metal matrix composites typically have better high temperature resistance than composites based upon polymers, and have better high temperature oxidation resistance than carbon-carbon composites.
The most widely known composite system is tungsten carbide used in many cutting tools. The composite is formed by dispersing fine tungsten carbide powder within a cobalt metal binder. This composite is successful because of its excellent physical properties, such as hardness, wear resistance, and toughness, as well as ease of manufacture. In the manufacturing process, molten cobalt wets the tungsten carbide powder. The wetting of the tungsten carbide is essential to the process of turning cold-pressed porous parts into dense, tough cutting tools.
In the area of metal matrix composites reinforced with fibers, the reinforcing fibers are typically either metals or ceramics The particular advantage of fiber reinforcement is that they can be aligned in the composite to improve strength, stiffness, wear resistance, hardness, thermal expansion coefficient, or thermal conductivity in the places and directions which are needed. Examples of metal fibers include tungsten, molybdenum, steel, and boron. Ceramic fibers which are currently in use include graphite, silicon carbide, alumina.
B. Metal Fiber Reinforced Composites
Unlike most ceramic fibers, metal fibers are generally wetted by the metal matrix in which the fiber is being incorporated. As a result, conventional casting techniques can be used to infiltrate a bundle of metal fibers. For example, copper and silver have long been known to wet refractory metals such as tungsten and molybdenum. Some of the early rocket nozzles which were used in the 1960's consisted of a porous tungsten infiltrated with silver; the silver evaporated from the surface of the nozzle, cooling the nozzle in the process. More recently, researchers have been working on a tungsten fiber reinforced copper matrix composite and on iron-based superalloys reinforced with tungsten fibers for high temperature applications.
One of the major problems with metal fibers reinforcing a metal matrix being encountered by researchers is the potential reaction and degradation of the metal fibers by the matrix material. For example, in attempting to reinforce nickel superalloys with tungsten fibers, the tungsten reacts with the matrix causing the tungsten to recrystallize and lose much of its strength.
In metal systems, it is desirable to have a strong bond between the fibers and the matrix. However, many of the reactions which cause bonding also cause degradation of the fibers. Fiber degradation significantly reduces some desirable fiber properties such as strength or stiffness, thereby defeating the purpose for using the metal fibers in the composite.
C. Ceramic Fiber Reinforced Composites
Unlike metal reinforcements, most ceramic reinforcements are not wetted by molten metals. In order to produce ceramic fiber reinforced composites, the most common approach is to infiltrate the fibers with the molten metal matrix under high pressure. In this case, fiber wetting is unnecessary, and bonding between the fiber reinforcement and the metal matrix is strictly physical. This process is often referred to as "squeeze casting."
From an engineering standpoint, handling molten metal under high pressure is difficult. A major problem with high pressure consolidation or squeeze casting is that two fibers pressed close together form a capillary which tends to exude molten metal. If a series of fibers are close together, the resulting voids between the fibers can degrade transverse strength such that the fibers lose their ability to withstand transverse stress.
In addition, because the fibers are not wetted by the metal matrix, only mechanical forces couple the fibers to the metal matrix. These "bonding" forces are weak because there is not atom-atom covalent bonding.
Some manufacturers currently roll metal powders into graphite fibers to produce fiber-metal tapes. A variation of this approach is used by others, in which copper electroplated on graphite fibers are hot pressed into simple shapes. Further research is currently focusing on aluminum-based composites using the high pressure metal approach.
A fundamental problem with high pressure consolidation is that large parts can only be formed with difficulty. For example, the recommended consolidation cycle for the copper plated graphite fibers is 1000 psi pressure at 750.degree. C. for 20 minutes.
Another significant problem with copper/graphite composites formed by high pressure consolidation is that the copper dewets from the fiber surface upon exposure to high temperatures. Hutto et al., "Development of Copper-Graphite Composites from Metal Coated Carbon Fibers," 31st International SAMPE Symposium 1145-1153, Apr. 7-10, 1986. This dewetting is graphically illustrated in FIG. 1 which shows molten copper beading on a graphite fiber. The beading is evidence that molten copper does not wet graphite.
The alternative to high pressure consolidation is infiltration of the reinforcing fibers with molten metal. In this case, wetting of the fibers by the molten metal is necessary. If the metal wets the fibers, the capillary action of the fibers draws the metal into the fiber preform, making it feasible to use techniques similar to metal casting to produce large scale quality composites. The technical background for producing metal matrix composites by liquid infiltration was recently reviewed by Delannay et al., "The Wetting of Carbon by Copper and Copper Alloys," Journal of Materials Sciences 149-155 (1987). An additional advantage of liquid metal infiltration is chemical bonding between the fiber and the matrix (as opposed to mechanical bonding in squeeze casting), potentially leading to improved physical composite properties.
In order to infiltrate the reinforcing fibers with molten metal, the fibers must be wet by the molten metal. As discussed above, ceramic fibers are generally not wet by molten metals of the type used as a composite matrix. There are two approaches which have been suggested in the art for improving the wettability of ceramic fibers. The first attempted solution to the problem is to coat the ceramic surface with a chemical layer which promotes wetting. The other approach which has been attempted by those in the art has been to develop an alloy matrix material which will wet the ceramic fiber.
Ceramic fibers are sensitive to surface damage; thus, if the metal matrix reacts with the fiber, then serious loss of fiber strength is likely to result. To prevent reactions between the metal matrix and the fiber, as well as to improve bonding, those skilled in the art have attempted to chemically coat the fibers. Various known coating techniques are discussed below.
Lightweight metals, particularly magnesium, aluminum, and titanium are of interest as matrix materials because of their low density. Wetting of graphite fibers in a magnesium metal matrix has been enhanced by coating the graphite fibers with silica. The magnesium reacts with silica which causes the fibers to be wetted. Fortunately, magnesium does not form stable carbides so there is no reaction between the graphite fibers and the magnesium matrix which would reduce the strength of the graphite fiber.
Current efforts to reinforce aluminum have involved attempts to incorporate silicon carbide fibers, graphite fibers, or alumina fibers into an aluminum matrix. Silicon carbide is thermodynamically unstable in contact with aluminum due to the following reaction: EQU SiC+Al &lt;=====&gt;Al.sub.4 C.sub.3 +Si
If the aluminum alloy contains 8% to 12% Si metal (depending on the temperature of the molten alloy), the decomposition of the silicon to form aluminum carbide is prevented.
When alumina is the fiber, there is a problem because alumina is not wetted by molten aluminum. However, wetting has been enhanced by treating the alumina fiber surface with silica. It has also been found that alumina may be wetted by an aluminum alloy which contains magnesium. Since both alumina and silicon carbide are electrical insulators, composites made with them do not suffer from electrochemical corrosion. For example, in recent years, Toyota has been manufacturing composite aluminum diesel pistons by high pressure liquid metal infiltration of an alumina-silica fiber preform.
There has also been substantial current research in preparing aluminum/graphite composites. Early processes included treating the graphite fibers with molten sodium to enhance wetting. Unfortunately, molten sodium degraded the graphite fibers. Hence, those skilled in the art have tried to coat the graphite fibers by chemical vapor deposition ("CVD") with a titanium boron coating prior to liquid metal infiltration by the aluminum. However, the titanium boron coating is not air stable. Even short exposure to air causes the titanium boron coating to lose effectiveness in enhancing wetting of molten metals.
Other researchers have examined coatings on graphite to produce copper/graphite composites. While nickel coatings have been wetted by copper, the copper readily dissolves the nickel layer. Dissolution of the nickel layer adds impurities to the pure copper, thereby significantly reducing conductivity. Moreover, molten copper dewets from the graphite fiber surface so that the copper matrix is not continuous. This dewetting of copper from the graphite fiber prevents the formation of true copper/graphite composites.
Other researchers have attempted to wet graphite fibers by using metal alloys as the matrix metal. Gwen L. Stahl, "The Influence of Alloy Additions on the Wettability of Graphite by Copper," Metallurgical and Welding Engineering Dept., Calif. Polytechnic State Univ., San Luis Obispo, Calif., (unpublished manuscript, October 1985). Stahl found that a twenty percent (20%) titanium copper alloy would wet silicon carbide coated fibers, but that a ten percent (10%) titanium copper alloy produced no wetting of the silicon carbide coated fibers.
In addition, it has been found that the alloy components must be carefully selected to avoid reacting with the fibers resulting in fiber degradation. For example, alloy components which promote fiber wetting often react with the fiber causing fiber degradation, thereby limiting the practical usefulness of the metal matrix alloy.
Titanium boron coatings on graphite fibers, discussed above, have been wetted by molten copper. Nonetheless, the coating must not be exposed to air prior to infiltration. This undesirable feature renders titanium boron coatings impractical for many industrial scale applications.
D. Comparison of Ceramic and Metal Reinforcements
If has been found that metal reinforcements can be more easily wetted by molten metals than can ceramic reinforcements. Also, the metal wires tend to have greater ductility than ceramic reinforcements. Ceramic reinforcements, however, have several key advantages over metal reinforcements.
Ceramic fibers have similar strength levels to metal fibers, but ceramic fibers have densities one half to one tenth (0.5 to 0.1) of metal fibers. This results in specific strengths between two (2) and ten (10) times greater than metal fibers.
Moreover, ceramic fibers, particularly those made of graphite, have a higher elastic modulus than metal fibers, resulting in stiffer composites. Ceramic fibers have lower coefficients of thermal expansion, thereby resulting in composites which have more stable dimensions when subjected to thermal cycling. The pitch-based graphite fibers have very high thermal conductivity along the fiber axis, enabling composites to be produced with superior thermal properties. Finally, ceramic fibers, particularly those based upon graphite, have high temperature properties which are superior to those of other reinforcements.
Based upon these properties, a preferred fibrous reinforcement would have the ease of manufacture of metal fibers (wettable by molten metal) and the density and strength properties of ceramic fibers.
From the foregoing, it will be appreciated that what is needed in the art are metal matrix coated fiber composites and methods of manufacturing such composites in which the fibers are not degraded during processing, the process and various components are stable in air, the process operates at atmospheric pressure, and the process in amenable to standard metal casting techniques.
Additionally, it would be a significant advancement in the art to provide metal matrix coated fiber composites and methods of manufacturing such composites in which the fiber is coated with a refractory metal layer (1) which is readily wetted by the metal matrix, (2) which is substantially inert to the fiber, (3) which would have a higher melting point than the metal matrix so that the coating itself does not have to wet the fiber and does not decrease the temperature range over which the resulting composite can operate, (4) which has little or no solubility in the metal matrix, both to maintain the desired chemical properties of the metal matrix, as well as to prevent gradual dewetting of the fibers by slow degradation of the coating, (5) which has a strong bond to both the fiber and the metal matrix to prevent fiber pull-out or other degradation of the physical properties of the resulting composite, and (6) which is stable in air.
It would be another advancement in the art to provide metal matrix coated fiber composites and methods of manufacturing such composites in which the fiber is coated with a barrier layer (1) which is inert to the fiber, (2) which resists diffusion through or alloying with the refractory metal layer and inhibits diffusion of the refractory metal therethrough, and (3) which has good bonding between the fiber and the refractory metal in order to maximize the physical properties of the composite.
Such metal matrix coated fiber composites and methods of manufacturing such composites are disclosed and claimed herein.